F.m. multiplex stereophonic broadcast receiver



June 20, 1967 RYOKICHI MU ET AL 3,327,059

F.M. MULTIPLEX STEREOPHONIC BROADCAST RECEIVER Filed Oct. 17, 1963 2Sheets-Sheet 1 L+R m W A m I 1! HIGH FREQUENCY AND fl 5 INTERMEDIATEFREQUENCY I AMPLIFIER F.M. DEMODULATOR 40 43 RIGHTIGNAL AMPLIFIER 4501147 4! 42 T FREQUENCY DISCRIMINATOR V 44 I 46 48 DEEMPHASS CIRCUI'T 62(LEFT SIGNAL AMPLIFIER RECTIFIER J +1501! I .54 BAND PASS a! Gm i FILTERAMPLIFIER 5 6V HIGH FREQUENCY AMPLIFIER F J 1 5% LOW PASS FILTER 8 l4IHZ 3 '6 FREQUENCY DISCRIMINATOR BAND PASS FILTER OSCILLATOR/ MULTIPL'ERJune 20, 1957 yo g M A ET AL 3,327,059

F.M. MULTIPLEX STEREOPHONIC BROADCAST RECEIVER 2 Sheets-Sheet F'iledOct. 17, 1963 L K M M W Jill T JET m m I." u 00% m0] INVENTOR M A M u 16020 Suz 1,061

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ATTORNEYS United States Patent 3,327,059 F.M. MULTIPLEX STEREOPHONIQBROADCAST RECEEVER Ryokichi Imamura, Tokyo, and Koro Suzuki, Hanno,Japan, assignors to Old Electric Industry Company Limited, Tokyo, Japan,a corporation of Japan Filed Oct. 17, 1963, Ser. No. 316,814- Claimspriority, application Japan, Feb. 5, 1963, 38/ 4,595 1 Claim. (Cl.17915) This invention relates to demodulators for F.M. multiplexstereophonic broadcast receivers and to stereophonic broadcastreceivers.

An object of the present invention is to improve the degree ofseparation of signals in stereophonic broadcasts.

Another object of the present invention is to prevent the generation ofbeat sounds and to elevate the degree of separation of right and leftsounds.

A further object of the present invention is to use an ordinary F.M.demodulating circuit instead of the demodulating circuit forstereophonic broadcasts in the case of using the stereophonic broadcastreceiver for single-dimensional broadcasts.

In the accompanying drawings.

FIGURE 1 is a circuit diagram of a demodulator for a conventional F .M.receiver.

FIGURE 2 is a view showing frequency spectra of signals demodulated bythe frequency discriminator in FIG- URE 1.

FIGURE 3 is a circuit diagram showing an embodiment of the demodulatoraccording to the present invention.

FIGURE 4 is a circuit diagram showing another embodiment of thedemodulator according to the present invention.

FIGURE 5 is a view showing a demodulating circuit in the case ofreceiving single-dimensional broadcasts in a receiver for stereophonicbroadcasts.

The present invention shall be detailed with reference to theaccompanying drawings.

FIGURE 1 shows a demodulating circuit for a conventional F.M. multiplexstereophonic broadcast receiver. 1 is a high frequency amplifying partof the receiver. 2 is a frequency discriminator. 3 is a low-pass filter.4 is a filter for removing contract broadcast bands. 5 is an oscillator,6 is a multiplier. 7 is a band-pass filter. As

thecircuit is wired as illustrated, the oscillator 5 is synchronized bya signal of 19 kc., and higher harmonics jjust twice as large are formedby the multiplier 6 and are added to demodulating diodes or rectifiers 8and 9. On the other hand, a contract broadcast frequency of 67 kc.,unnecessary for F.M. stereophonic broadcasts, is removed by means of thefilter 4, L+R sound parts of 19 kc. and 0-15 kc. are removed by means ofthe band-pass filter 7 and only a signal of 38 kc.i(LR) is taken out andis added to the diode circuits 8 and 9 as in the diagram. However, asshown in FIGURE 2, the (LR) signal will be in the form of modulationsseparated from the carrier wave of 38 kc. after the 38 kc. carrier wavehas been amplitude modulated by the (LR) signal. Therefore, when exactlythe same frequency as the original 38 kc. is applied to the (LR) signalcircuit as in FIGURE 1 and is demodulated, the original low frequencysignal of (LR) will come to appear on the output side of said diodes 8and 9. If this (LR) signal and the (L-l-R) signal, which has passedthrough the low-pass can be derived from the terminals 18 and 19,respectively. When they are properly amplified and are led to speakers,broadcast sounds can be heard from the right and left speakers,respectively.

In the circuit mentioned above, in case there is no L signal, if the Rsignal appears at the terminal 18 of the L signal, the right and leftsignals will not be discriminated from each other and the stereophonicbroadcast will be meaningless. Therefore, the ratio of the R signalappearing at the terminal 19 to the R signal appearing at the terminal18, in case there is no L signal, is called the degree of separation ofthe R signal. The smaller this value, the better.

The object of the present invention is to improve the above mentioneddegree of separation. That is to say, while feeding the R signal onlyand measuring the R signal voltage leaking to the L signal terminal 18,the respective circuits are adjusted to minimize this value. It has beendiscovered that, even in case the respective circuits are reasonablyadjusted so that the theoretical leakage of the R signal may besubstantially zero, the leaking R signal voltage at the terminal 18 willbe very large. In scrutinizing this mysterious phenomenon, it has beendiscovered that, when the L+R signal, coming through the low-pass filter3 and resistors 16 and 17, and the 38 kc. output multiplier 6 are mixedtogether by means of the diodes 8 and 9, beat sounds will be generatedand will appear at the L terminal 18. For example, in case there is no Lsignal but there is only the R signal as described above, if the Rsignal frequency coming through the lowpass filter 3 is made 6.333 kc.,just six times as large as it will be 38 kc. and, when these twocurrents are mixed together by means of the diodes 8 and 9, the beatsounds will appear at the terminals 18 and 19. Not only will thisphenomenon reduce the apparent degree of separation but also thegenerated beat sounds will be of a frequency quite different from of theoriginal signal and therefore, for example, in a music broadcast, asound which is quite different from the broadcast sound will be mixed inand the tone quality of this receiver will be greatly deteriorated. Inmeasuring commercial F.M. receivers in which the degree of separation isas low as to be, for example, about 5 to 10 db, it is found in all ofthem that the output of the beat has become far larger than the leakageof the substantial L or R signal. Therefore, if this beat sound isremoved, the degree of separation can be remarkably improved. I

The objective of the present invention is to prevent the generation ofsuch obstructing beat sounds and to improve the degree of separation ofthe right and left sounds of an FM. stereophonic broadcast receiver. Forthis purpose, as evident from the above described explanation, the L+Rsignal must be prevented from being applied to the diodes 8 and 9without obstructing the inherent function of the demodulator. Thus, aninterference preventing device comprising transistors or vacuum tubes isprovided between the diode circuit, for demodulating the LR signal of 38kc. and taking out the LR signal of a low frequency composition, and thematrix circuit for combining the demodulated LR signal and the L+Rsignal.

An embodiment of the demodulator according to the present invention isshown in FIGURE 3. In the drawing, 20, 21, 22, and 23 are signal voltagedividers. 24 is a balancing variable resistor for matching vacuum tubecharacteristics. 25 and 26 are vacuum tube amplifiers. The componentsdesignated by the other numerals are exactly the same as in FIGURE 1. Inthis invention, the LR signal is amplified by the vacuum tubes 25 and 26and is fed to the matrix circuit. On the other hand, the L+R 3 signal isfed to the matrix circuit in exactly the same manner as in FIGURE 1.Both signals are combined in said matrix circ-uit. It is exactly thesame as in the case of FIG- URE 1 that the signals of the sum anddifference or L and R signals are derived at the terminals 18 and 1?,respectively. But, in FIGURE 3, as the vacuum tubes 25 and 26 areinterposed between the diode circuits 8 and 9 and the L+R signal inputend, the L+R signal will not be applied to the diode through the gridsfrom the anodes of the vacuum tubes. That is to say, the two signalswill not be mixed in the diode circuit and will not generate beat soundsas described above. On the contrary, the LR signal will appear at theanode from the grids of the vacuum tubes and will be applied to thematrix circuit. That is to say, the vacuum tubes 25 and 26 in this casecan be considered to be acting as valves passing the signal from left toright in the drawing but preventing the signal from passing from rightto left. Further, as the anode voltage of the vacuum tubes 25 and 26 ishigh enough and the anode voltage-current characteristics are linear, asdifferent from the circuits of the diodes 8 and 9, there is no fear ofgenerating beat sounds in this part. According to the present invention,the conventional degree of separation of about db can .be easilyelevated to be higher than -30 db. Even if transistors are used in placeof the vacuum tubes, exactly the same effect can be obtained.

In the above mentioned embodiment, by using vacuum tubes or transistors,the passage of the current in the reverse direction, and theinterference of the oscillator current of 38 kc. with the L+R signal,are prevented. However, such interference preventing device is notlimited to vacuum tubes or transistors. In short, the device may be suchas will prevent the L+R signal from being applied to the diodes 8 and 9and will effectively apply the LR signal to the matrix circuit.

In FIGURE 4 is shown another embodiment of the present invention whereinthe characteristics of a rectifier of passing signals in one directiononly are used for such purpose. In the drawing, the numerals 8 to 19designate the same parts in FIGURE 1, but diodes or rectifiers 31, 32,33 and Marc inserted as illustrated. If the polarities of the diodes orrectifiers are connected as illustrated, the L+R signal coming throughthe resistors 16 and 17 will be blocked by the diodes 33 and 34 and willnot be applied to the diodes -8 and 9, respectively, when its phase inplus. When the phase of the L+R signal is minus, the signal-will passthrough the diodes 33 and 34 but will be shorted by the diodes 31 and 32and will not be applied to the diodes 8 and 9. On the contrary, the LRsignal will easily pass through the diodes 33 and 34 and -will not beshorted by the diodes 31 and 32.

Further, in an F.M. stereophonic broadcast receiver, by using acomplicated demodulating circuit, the right and left sounds of receivedsignals are demodulated to operate the right and left speakers,respectively. In the present invention, in the case of a singledimensional broadcast, the demodulating circuit for stereophonicbroadcasts is not to be used but an ordinary F.M. demodulating circuitis to be used. The present invention provides a receiver for F.M.stereophonic broadcasts wherein a low frequency amplifyingcircuit isswitched over by using a 19 kc. pilot signal included in F.M.stereophonic broadcasts.

According to the present invention, in view of the fact that, in thecase of stereophonic broadcasts, the 19 kc. pilot signal is always beingsent, by operating a relay with the energy of said signal, thedemodulating circuit for stereophonicbroadcasts may be automaticallyswitched in atthe time of stereophonic broadcasts, and the demodulatingcircuit for single-dimensionalbroadcasts may be automatically switchedin at the time of single-dimensional broadcasts instead of stereophonicbroadcasts.

In FIGURES, an antenna 40 is connected to a frequency discriminator 42through a high frequency and intermediate frequency amplifier 41 of anF.M. receiver.

Said discriminator 42 is connected to a demodulator 43 for F.M.stereophonic broadcasts and a de-emphasis circuit 44. Demodulator 43 hasterminals a and a, and the de-emphasis circuit 44 has terminals 1) andb. Terminals a and b can be connected to a right signal amplifier 45connected with a right speaker 47 through a later described contact 49,Also the above mentioned terminals 11 and b can be connected to a leftsignal amplifier 46 connected with a left speaker 48 through a laterdescribed contact 49. The above mentioned demodulator 43 for F.M.stereophonic broadcasts is connected to a B+ source of v. through thelater described contact 49".

On the other hand, the output of the frequency discriminator 42 isconnected to a series circuit of a band-pass filter 50 for 19 kc.signals, 19 kc. signal amplifier 51, rectifier 52 and relay 53. Further,a pilot lamp 54 for indicating stereophonic broadcasts and a pilot lamp55 for indicating single-dimensional broadcasts are connected to 6 v.source through a later described contact 49". The above mentionedcontacts 49, 49, 49 and 49 are operatively connected contacts for saidrelay 53, and will be kept in the illustrated state when said relay 53operates.

The present invention as illustrated operates as follows. During asterephonic broadcast, as the 19 kc. pilot signal is being sent, saidsignal will be taken outby the band-pass filter 54 will be amplifier bythe 19 kc. amplifier 51, will operate by the relay 53 by means of therectifier 52 and will keep the contacts 49, 49, 49" and 49" in theillustrated positions. As a result, the output of the sterephonicbroadcast will be amplified by the amplifiers 4S and 46 through thecontacts a and a. On the other hand, in case the broadcast frequency isof a single-dimensional broadcast, as there is no 19 kc. pilot signal,the relay 53 will not operate, therefore all the contacts 49, 49, 49"and 49 will be switched over and the broadcast frequency will be ledinto the amplifiers 45 and 46 through the frequency discriminator 42 andde-emphasis circuit 44 in the same manner as in an ordinary F.M.receiver. As a result, in the case of a single-dimensional broadcast, asthe complicated demodulator 43 for stereophonic broadcasts is not usedin the circuit formation, the tone quality will not be reduced and afavorable tone quality be enjoyed. Further, as there is a 38 kc.oscillator, having a considerably large output, for demodulating theright and left signals in the stereophonic broadcast demodulator 43, inorder to prevent the generation of heat sounds by the leakage of theoscillator output, by cutting off the B+ source with the contact 49" inthe case of a single-dimensional broadcast in which said demodulator 43is not used, a good tone quality can be secured. Further, bysimultaneously using the pilot lamps 54 and 55, whether the broadcastnow coming out of the speaker is a single-dimensional broadcast or astereophonic broadcast can be easily identified.

What is claimed is:

An F.M. multiplex stereophonic receiver for deriving left (L) and right(R) audio signals transmitted on a carrier wave as modulations of asub-carrier in the form of sum (L+R) signals and difference (LR)signals, said receiver comprising, in combination, a frequencydiscriminator; means applying the modulated carrier wave to saidfrequency discriminator; low pass filter means connected to the outputof said frequency discriminator and passing the L+R signals; band passfilter means connected to the output of said frequency discriminator andpassing the LR signals; an L signal output terminal; an R signal outputterminal; frequency generator means providing a sub-carrier frequencyfor the LR signal; combining means connected to said band pass filtermeans and to said generator means and mixing said sub-carrier frequencywith said LR signal; means connecting the output of said low pass filtermeans to said output terminals to apply said L+R signals to said outputterminals; diode means, including plural diodes, connected to saidcombining means and deriving the LR signals; and rectifier meansconnecting said diode means to said output terminals and providing forapplication of said L-R signals from said diode means to said outputterminals while blocking application of said L+R signals to said diodemeans; said diode means Comprising a pair of first diodes each connectedbetween said combining means and a respective output terminal; saidrectifier means comprising a pair of second diodes having their anodesinterconnected to ground and each having a cathode connected to arespective output terminal; said rectifier means further including athird pair of diodes each connected in a forward direction between thecathode of a respective second diode and the corresponding outputterminal.

References Cited UNITED STATES PATENTS Dome 179-15 Frank 179-15 VonRecklinghausen 179-15 Bially 179-15 Limberg 179-15 De Vries 179-15 10DAVID G. REDINBAUGH, Primary Examiner.

ROBERT L. GRIFFIN, Examiner.

